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TOWARD THE END of the last ice age—between 15,000 and 18,000 years ago—one of the most cataclysmic events in Pacific Northwest history reshaped the upper Willamette Valley. As a result, it gave certain areas of modern-day Portland a lung cancer risk that is second only to smoking.

The culprit is radon. It’s an odorless, colorless radioactive gas emitted by all types of rocks, but especially granite. It becomes harmful when it seeps up through the soil and into residential basements. Every year, 20,000 people in the United States die of radon-caused lung cancer, according to the U.S. Environmental Protection Agency.

Portland State geology professor Scott Burns has spent the past 20 years mapping the radon levels of locations throughout the Portland metro area. His first study in 1993 took radon data from 1,100 Portland homes, and mapped low, moderate and high levels of risk. The study changed the way government officials, homeowners, builders, and real estate agents looked at radon, and eventually helped to create new building laws to guard against radon exposure.

That initial study turned out to be the tip of the iceberg. In January, Burns and his student research team came out with a report from a much broader sampling: 32,000 homes in the Portland area and another 22,000 from around the state. It showed more places in the Portland area with a high risk of radon exposure, and for the first time, pointed out other hot spots in the state with dangerous levels of radon, including Milton-Freewater, Lakeview, Sandy, Silverton, and parts of Baker County.

Suddenly Burns was on the front page of Oregon newspapers and was being interviewed by radio and TV reporters. Not long after, Salem’s Statesman Journal reported hardware stores in Silverton were having a hard time keeping home radon test kits on the shelves. That’s because Burns wasn’t just talking about the newfound prevalence of radon, but the fact that it’s something homeowners can do something about. Homeowners can perform an inexpensive test in their homes, and if levels are high, they can fix the problem by repairing basement walls and installing ventilation.

“It’s the cheapest of all the geological hazards out there,” he says.

But how did it get here?

MOST OF THE RADON danger in the Portland area and, in fact, much of Portland’s topography, can be traced to the Missoula Floods, which occurred several thousand years ago. During the last ice age, a glacier in Idaho blocked a major river coming out of Montana. Water behind the blockage rose to a depth of 2,100 feet and contained the equivalent of half of Lake Michigan. The ice dam broke, creating a flood of biblical proportions. It scoured the eastern half of Washington, then filled up the Columbia Gorge, traveling at speeds of up to 60 miles an hour.

“If you take the flows of all the rivers in the world measured in cubic meters per second, add them up and multiply by 10, you have the flow rate of the Missoula Floods,” Burns says.

The floodwaters filled the Willamette Valley to a depth of 400 feet all the way to present-day Eugene, and then receded north, joined the Columbia River, and drained into the Pacific.

The water carried granite sediment from Montana, and deposited it at various spots along its course. All rocks are radioactive to some extent, because they all contain trace levels of uranium, but some have more than others. Basalt, coming from the Northwest’s volcanoes, doesn’t have much; granite does. Radon is a product of the breakdown of that uranium. A house built on soils containing granite sediments—especially porous soils that allow the radon gasses to rise and dissipate—is subject to dangerous levels of radon exposure.

Portland’s Alameda Ridge is one such place, and has one of the highest radon levels in the city. The ridge was formed when the Missoula floodwaters encountered Rocky Butte. There, the waters slowed and changed course, depositing several cubic miles of granite-rich sediment.

Burns has known about radon levels on Alameda Ridge since his first study came out 20 years ago, and his most recent study backed it up. The surprises came when areas that were not really affected by the Missoula Floods cropped up as being high in radon: Sandy, for example.

“Sandy is my mystery city. It was above the Missoula Floods, so I can’t figure out why the levels are high.”

Newberg also doesn’t fit the same profile as other high-radon areas. The soils there are not very porous—which seems to be a prerequisite for high readings. However, it sits on top of major faults, which enable the release of radon from deep in the earth, Burns says.

DATA FOR THE STUDY came from tests people throughout the state performed in their homes. For as little as $12, you can get a test kit from your local hardware or home improvement store. You just open the kit and place it in your basement. Material in the kit reacts to radon in the air. After a week, you send the kit to a lab, and the lab sends back a reading, measured in picocuries per liter. Anything over 4 picocuries per liter is considered high. Testing is included in the price of the kit.

For $25, you can get a longer-term, more accurate test kit. It’s made of a different material, and it’s placed in a home for three months.

“It gives you a better picture of what’s going on,” Burns says.

Brett Sherry, the state radon officer with the Oregon Health Authority gathers test results from the manufacturers of the kits, and that’s where Burns got the data for the recent mapping study. Sherry says his office is too small to do the mapping work. As a result, Burns plays a valuable role in the state’s effort to get the word out about radon.

Burns enlisted students Tamara Linde, Kassie Lindsey, and Hilary Whitney to crunch the test numbers, sort them by zip code, and plot the results on a map. The experience was eye-opening for Whitney, who talked with the owners of private radon testing companies in Portland. Their test results showed that one house could test radically different than one next door. The difference is often the result of the home’s condition. A home with cracked basement walls that let in radon gas can have a much higher reading than one with no cracks, even if they’re right next to each other.

“So the risk of being exposed to radon is really not guaranteed to be distributed evenly in a given neighborhood or area, and that’s why the EPA, local radon testing companies, and those of us who are studying radon at PSU recommend that every home be tested for radon gas. Because you never know,” Whitney says.

Burns couldn’t agree more. He says the data his team gathered showed that Portland in general had twice the national average of homes with radon levels higher than 4 picocuries per liter, and that it’s in everyone’s best interest to do a home test.

“We will probably generate another 20,000 to 30,000 tests in the Portland area as a result of our findings,” he says.

John Kirkland, a staff member in the PSU Office of University Communications, wrote “Stop-Motion Magic” in the winter 2013 Portland State Magazine.